A plural objective microscope includes, among other structure, a rotatable nosepiece to which is secured a series of objectives of different power. Objective, as used herein, includes, but is not limited to, the "semi- objectives" such as disclosed in U.S. Pat. Nos. 3,481,665 and 3,583,789. The provision of a plurality of objectives permits the microscope operator to selectively bring any one of the objectives into alignment for use in viewing a slide, or other object, upon which the optical system is to be focused.
Typically, an objective includes a generally tubular shaped housing having at one of its ends a series of threads adapted for engagement with mating threads provided on the rotatable nosepiece. The objective housing also includes, adjacent to the threaded portion thereof, a shoulder which, when the objective is secured to the nosepiece, is in engagement with a mating shoulder or surface provided on the nosepiece. A lens, or series of lenses, is positioned within the objective housing by appropriately configured and dimensioned lens supporting cells and spacers.
In higher power objectives, which are generally longer than lower power objectives and have very small working distances, the housing may be formed of two generally cylindrically shaped members. The first or outer member includes the threaded end-cooperating shoulder arrangement for receiving and positioning the objective relative to the rotatable nosepiece. The second or inner member, which supports the lens elements, is partially and slidably received within the outer member. In such an arrangement the inner member is provided with a shoulder which is held in engagement with an inwardly extending flange, provided on the outer member, by means of a spring member. This arrangement permits the inner member to retract, against the pressure of the spring, into the outer member and, thereby, prevent damage to either the specimen or the objective during focusing within the small working distance.
In a series of objectives such as those carried by the usual rotatable nosepiece of a microscope it is desirable that all the objectives be selectively positioned to view an object without changing the focus adjustment of the microscope. The object, therefore, remains in focus for all the objectives provided on the nosepiece without changing the relative position of the object to the nosepiece or the nosepiece to the eyepiece. When the series of objectives mounted on the nosepiece functions in this manner, they are said to be parfocal.
To achieve parfocality, the most generally accepted procedure is to machine the shoulder provided adjacent the threaded end of the objective housing. This alters the position of the lens system carried by the housing relative to both the stage and the eyepiece of the microscope. This is a generally tedious and time consuming operation which can only be done satisfactorily at the factory. Another generally accepted procedure to achieve parfocality, usable either by itself or in combination with machining, is the addition or subtraction of spacers between the shoulder of the objective and the mating shoulder or surface provided on the nosepiece. In either case, once parfocality has been achieved for a series of objectives on the nosepiece, they were locked into that position and the entire assembly sent to the ultimate user.
Alternate ways of achieving parfocality for a series of objectives are disclosed in U.S. Pat. Nos. 1,557,503 and 1,889,794. In U.S. Pat. No. 1,557,503 parfocality is achieved by forming the objective housing in two parts which are adjustable relative to each other by means of a series of threads. A sleeve, the correct length of which is determined at the time of assembling the objective, is inserted inside the objective to fix the position of the two parts of the housing relative to each other. The objective disclosed in U.S. Pat. No. 1,889,794, like the objective disclosed in U.S. Pat. No. 1,557,503, includes a two part housing. Parfocality in this case is achieved by selectively rotating the two parts of the housing relative to each other to obtain the correct position. This position is then fixed by use of an inner cylindrical spacing member in cooperation with a retaining nut threadably received within the housing to hold all the parts in the desired position.
Another method of achieving parfocality is particularly adapted to higher power objectives wherein the objective housing is composed of two generally tubular members and wherein one of the tubular members is slidably received within the other tubular member. In place of the fixed flange or shoulder normally provided on the outer member, against which a shoulder provided on the inner member is held in engagement by means of a suitable spring member, an adjustable collar is threadably received in the lower end of the outer member. With this arrangement, parfocality is achieved by rotating the collar relative to the outer member which, in turn, determines the non retracted position of the inner member relative to the outer member. Collar rotation is affected by a special spanner. Once adjusted, the collar is locked in place by a suitable cement to prevent rotation and, hence, loss of parfocality. Cementing is necessary to prevent collar rotation induced by repeated engagement of the shoulder on the inner member against the stop surface provided on the collar.
The above-described methods of achieving parfocality have the disadvantage in that parfocality is set at the factory and cannot easily be adjusted in the field by the ultimate user. Thus, replacement of objectives is best handled by the factory or, in the field, by specially trained personnel. U.S. Pat. No. 2,195,657 describes apparatus which permits setting and/or adjusting parfocality in the field without the use of spacers, machining, or partially disassembling the objective as is required by the previously described objectives. The apparatus disclosed in this patent includes an objective having a two part housing, the upper part being adapted to engage with the nosepiece of the microscope and the lower portion carrying the described lens assembly. The two portions of the housing are rotatable relative to each other and are locked in position relative to each other by means of a set screw provided on the side of the objective housing. Parfocality can be set or readjusted by loosening the locking screw and rotating the lower housing member relative to the upper housing member to the desired position and then retightening the locking screw to hold the adjusted position.